Final answer:
During digestion, acid, digestive enzymes, and bile are secreted into the lumen of the digestive tract to aid in breaking down food, while hormones regulate digestion but do not enter the digestive lumen.
Step-by-step explanation:
All of the following are secreted into the lumen of the digestive tract during the process of digestion except hormones. The substances secreted into the digestive tract primarily include acid, digestive enzymes, and bile. These secretions enter the gastrointestinal (GI) tract to facilitate the breakdown of food. Hormones like gastrin, secretin, and cholecystokinin (CCK) regulate aspects of digestion, such as which digestive secretions are released and when. However, hormones are not secreted into the digestive lumen; they exert their effects via the bloodstream on their target organs.